The present invention relates to methods and apparatus for improving the performance of multimode optical fibre communications systems, and in particular to methods and apparatus for launching optical radiation into multimode fibre.
In the late 1970s, and early 1980s, much work was carried out to improve performance of multimode optical fibre communications systems. However, when multimode fibre was replaced by a singlemode fibre, as the medium of choice for use in high bit rate, long distance communications systems, much of this work ceased. Multimode fibre has continued to be used in optical communications for systems operating at lower bit rates, and over shorter distances, for example in building or campus LANs. There is thus a large installed base of multimode fibre, which represents a significant investment.
In recent years the demand for high data rate LANs has increased dramatically, for example to 1 GBit/s and beyond. Thus, even though multimode optical fibre is only utilised over short distances, for example 500 meters, the required data rates cannot be achieved utilising conventional techniques.
A key aspect in determining the bandwidth of a multimode optical fibre communications link, which has been recognised for many years, is the number and distribution of modes within the multimode fibre which are excited, and therefore carry optical energy. See for example Chapter 7 of "Optical Fibres for Transmission" by John E. Midwinter, published by John Wiley & Sons in 1979. If a pure low order singlemode is launched into a multimode fibre, and there is no mode mixing, the bandwidth and other characteristics of the optical communication link will be that of a single-mode fibre, i.e. the link will have high bandwidth. If mode mixing occurs, for example due to fibre profile irregularities, or mechanical perturbations of the fibre, energy will be coupled from the single lowest order mode into higher order modes having lower group velocities, and additional pulse dispersion will inevitably result, leading to a lower overall bandwidth for the communications system. Alternatively, if light is launched into the same multimode fibre in a manner so as to uniformly excite all modes of the multimode fibre (a so called "overfilled launch"), and if no mode mixing occurs, a maximum pulse spread will be seen, and the bandwidth of the communications system will be at a minimum. If mode mixing is introduced to this situation, because individual photons will then spend some time in many different modes, and will have travelled many short distances at different group velocities, less pulse spreading will be experienced. In the ideal case rather than experiencing an increase of pulse spreading which is proportional to the length of the optical communications link, pulse spreading builds up only in proportion of the square root of the length of the optical communications link. Thus, in the early 1980s, although various alternative schemes were investigated (see eg U.S. Pat. Nos. 4,050,782 and 4,067,642), it was generally accepted that it was desirable to launch many modes into a multimode optical fibre, and to ensure that adequate mode mixing occurred in order to achieve a reasonable, and predictable, bandwidth for an optical communications link.
Despite this practical approach, it was however theoretically predicted that if the number and distribution of modes excited within a multimode fibre could be precisely controlled, the bandwidth of the communications link could be improved. For example, see Section 7.6, page 126 of Midwinter's book where it is suggested that controlled mode coupling can be utilised to prevent coupling to the highest order modes of the fibre so as to increase the fibre bandwidth without incurring a loss penalty. Nevertheless, it is stated here that "It must be said, however that experimentally it looks extremely difficult to achieve such a precisely controlled fibre environment, and at the time of writing no reports of experimental testing are known."
In recent years lasers rather than LEDs (Light Emitting Diodes) have been utilised with multimode optical fibre communications systems. There are a number of reasons for this, firstly, lasers can be directly modulated at higher speeds than LEDs. Secondly, because lasers have only a few, well controlled, transverse modes they can be utilised to excite only a selected few modes of the multimode optical fibre. The aim has been to alter the launch conditions so as to increase the bandwidth of the multimode fibre beyond its OFL (overfilled launch) bandwidth. The OFL, conditions are specified in an EIA/TIA standard (EIA/TIA 455-54A "Mode scrambler requirements for overfilled launching conditions to multimode fibers") and are designed to ensure that all modes of the multimode fibre are uniformly exited. While an OFL launch ensures a certain minimum bandwidth, this worst case bandwidth is no longer sufficient and there is a requirement to reliably improve the bandwidth that can be guaranteed to be achievable for any multimode fibre system.
It is known to utilise a laser to launch a small exciting spot of radiation at the centre of a multimode fibre, for example by employing a singlemode fibre, or a lens between the laser source and the multimode fibre. The aim of such a "centre launch", as it will be termed herein, is to excite only the lowest order mode (or at the most a few low order modes) of the multimode fibre. As discussed above, if only a few modes of a multimode fibre are excited, and little mode mixing occurs, the bandwidth of a multimode optical fibre communications system can theoretically be increased dramatically since modal dispersion is effectively eliminated. However, there are several disadvantages to the use of a centre launch to increase the bandwidth of multimode communications systems. Tight tolerances on the position and size of the exciting spot are required to ensure the launch of only a single mode or a few low order modes. Secondly, as discussed above, if mode mixing or coupling occurs due to the fibre environment or at connectors within the communications system, many more modes will be excited despite the use of a centre launch. Thirdly, the centre launch technique is sensitive to defects in the multimode fibre refractive index profile and particularly sensitive to any "central dip" in the profile. Such a central dip can occur for example due to the evaporation of dopant from the inner surface of a fibre preform when the preform is collapsed during the fabrication of the fibre. Finally, the use of a laser to launch a small number of low order modes into a multimode fibre is known to give rise to modal noise. The modal noise problems experienced when using high coherence sources with multimode fibres are exacerbated when only a few modes of the multimode fibre are excited. Modal noise problems have received much recent attention, see for example "Improved Multimode Fibre Link BER Calculations Due To Modal Noise and Non-Self Pulsating Laser Diodes" R. J. S. Bates et al, Optical and Quantum Electronics 27 (1995) 203-224.
U.S. Pat. No. 5,416,862 discloses the use of singlemode fibre whose longitudinal axis is tilted at a predetermined angle to the longitudinal axis of a multimode fibre to launch a small number of higher order modes into the multimode fibre. While this angled launch is said to reduce modal dispersion (and thus increase bandwidth) and to reduce sensitivity to mechanical perturbations, the excitation of only higher order modes causes a considerable loss of optical power.